COAL TAR

Coal tar is a complex hydrocarbon mixture produced by thermal destruction (pyrolysis) of coal, typically a dark viscous liquid or semisolid with a smoky or naphthenic odor. The composition of coal tar will be influenced by the process used for pyrolytic distillation as well as by the original composition of the coal; however all coal tars will be comprised of a variable mixture of organic compounds including benzene, toluene, xylenes, cumenes, coumarone, indene, benzofuran, naphthalene, acenaphthene, methylnaphthalenes, fluorine, phenol, cresols, pyridine, picolines, phenanthracene, carbazole, quinolines, fluoranthene, and pyrene. The number of specific chemical constituents is in the thousands. Coal tar creosotes and coal tar distillates, oily liquids generally lighter in color and of lower viscosity than coal tar, are fractions produced by additional distillation of crude coal tar. Coal tar pitch is a highly viscous dark semisolid byproduct of coal pyrolysis. Coal tar volatiles are the vapors produced from heated coal tar or coal tar pitch, containing lower molecular weight (smaller ring number) polycyclic aromatic hydrocarbons (PAHs).
Coal tar is noteworthy as one of the first – if not the first – chemical substances documented to cause cancer through occupational exposures. In the eighteenth century, Sir Percival Pott, a British surgeon, noticed a higher incidence of cancers in chimney sweeps chronically exposed to soot and coal tar. He then demonstrated excess cancers occurring in laboratory animals when coal tar is applied to the ears and skin. In the early twentieth century, polycyclic aromatic compounds isolated from coal tar were identified as chemical carcinogens.

Coal tars are by-products of the destructive distillation (carbonization) of coal to produce coke or gas. The composition and properties of a coal tar depend primarily on the temperature of the carbonization and to a lesser extent on the nature (source) of the coal used as feedstock. In general, coal tars are complex combinations of hydrocarbons, phenols, and heterocyclic oxygen, sulfur, and nitrogen compounds. Over 400 compounds have been identified in coal tars, and as many as 10,000 may actually be present. The PAH content of coal tars increases with increasing carbonization temperature. Coal tars typically are black or almost-black viscous liquids or semisolids with a characteristic naphthalene-like odor (ATSDR 2002). They are slightly soluble in water, partially soluble in acetone, carbon disulfide, chloroform, diethyl ether, ethanol, methanol, petroleum ether, and sodium hydroxide, and soluble in benzene and nitrobenzene. Lowtemperature coal tars (formed at temperatures below 700°C) are black, viscous liquids that are denser than water and contain a lower percentage (40% to 50%) of aromatic compounds than high-temperature coal tars (IARC 1985). Coal tars are highly flammable and corrosive, and toxic gases may be released when they burn. Their vapors can form explosive mixtures with air (HSDB 2009).
Coal-tar pitches are shiny, dark-brown to black residues produced during the distillation of coal tars. They contain various PAHs, their methyl and polymethyl derivatives, and heteronuclear compounds (IARC 1985).

Coal tars and coal-tar pitches have many uses in industry and in consumer products. Coal tars are used primarily for the production of refined chemicals and coal-tar products, such as creosote, coal-tar pitch, and crude naphthalene and anthracene oils from the distillation of crude coal tar. Coal tar has been used as a fuel in open-hearth furnaces and blast furnaces in the steel industry, as a binder and filler in surface-coating formulations, and as a modifier for epoxy-resin surface coatings. U.S. Pharmacopeia–grade coal tar is approved for use in denatured alcohol (IARC 1985). Coal-tar preparations have been used for many years to treat various skin conditions, such as eczema, psoriasis, seborrheic dermatitis, and dandruff. Both prescription and nonprescription preparations are available and include cleansing bars, creams, gels, lotions, ointments, shampoos, and other topical solutions and suspensions (DermNet NZ 2010). Coal tar is also registered as an active ingredient in pesticides with the U.S. Environmental Protection Agency (EPA 2003).
Coal-tar pitches are used primarily as the binder for aluminumsmelting electrodes (IARC 1984). They are also used in roofing materials, to impregnate and strengthen refractory brick (for lining industrial furnaces), and in surface coatings, such as pipe-coating enamels and black varnishes used as protective coatings for industrial steelwork and as antifouling paints for boats. Hard pitch is used as a binder for foundry cores. Coke-oven pitch is used to produce pitch coke, which is used as the carbon component of electrodes, carbon brushes, and carbon and graphite articles. Distillation fractions and residues from high-temperature coal tars are used for road paving and construction and in the production of naphthalene, recovery of benzene, production of anthracene paste, briquetting of smokeless solid fuel, impregnation of electrodes and fibers, and manufacture of electrodes and graphite (IARC 1985).

Coal tar is used for constructing roads, electricity generation and coking. It has been used as topical medical treatment for psoriasis and dandruff and is formulated as a soap or ointment. It is also used for other types of rashes (such as eczema, atopic dermatitis, chronic exudative dermatitis).

Coal tar is primarily used as a raw material in the manufacture of plastics, solvents, dyes, and in the manufacturing of other chemicals. Most coal tar undergoes further distillation. Industries that use coal tar include road paving, roofing, smelting, and coking. Coal tar creosote is used as a wood preservative. Coal tar products are also ingredients in medicine (Coal Tar United States Pharmacopeia) used to treat skin diseases such as psoriasis or eczema.

The by-product from the destructive distillation of coal. Almost black semisolid. A complex combination of aromatic hydrocarbons, phenolic compounds, nitrogen bases and thiophene.

A human and experimental skin irritant. A flammable liquid. When heated to SYNS: ANTHMCITE PARTICLES 0 COAL FACINGS 0 COAL, GROUND BITUMINOUS (DOq 0 COAL-MILLED 0 COAL SLAG-MILLED SEA COAL decomposition it emits acrid smoke and irritating fumes.

Coal tars and coal-tar pitches are known to be human carcinogensbased on sufficient evidence of carcinogenicity from studies in humans.

Due to the variability in composition, it is not possible to describe all potential mechanisms of toxicity for coal tar. The reader is recommended to review additional references for individual constituents found in the Table of Contents. It is likely that acidity of some constituents, such as phenols, and the defatting potential of some hydrocarbons, contribute to the irritancy of coal tar. Phototoxicity of PAHs is likely to be a main cause of contact irritation.
The carcinogenicity of PAH constituents is believed to lie in their potential for their reactive metabolites to be bound to macromolecules such as DNA. The mechanism of therapeutic value as a topical agent in the treatment of skin diseases is unknown but is thought to involve decreased epidermal proliferation.

Environmental partitioning will vary dependent upon the chemical characteristic of various constituents of coal tar. Photochemical degradation may occur in the atmosphere. If entered into aquatic systems, light hydrocarbon constituents such as benzene will volatilize in the air. Biodegradation in aquatic ecosystems will occur at various rates for different constituents. Large molecules such as PAHs are likely to adsorb to soil and sediment, undergoing slow degradation. Other hydrocarbons, such as phenols, may be readily degraded under aerobic conditions.